1. Field of the Invention
This invention relates to a process for the after-passivation of phosphated metal surfaces.
2. Statement of Related Art
The protection of metal surfaces, particularly the protection of iron and steel surfaces, by phosphate-containing coatings has been known for some time. In this connection, a distinction is drawn between "nonlayer-forming phosphating", i.e. the use of alkali and/or ammonium orthophosphate solutions to produce iron phosphate coatings in which the iron ion emanates from the metal surface to be coated, and "layer-forming phosphating", in which zinc phosphate layers or zinc/calcium phosphate layers are formed on metal surfaces using zinc or zinc/calcium phosphate solutions.
Phosphate coatings such as these not only improve the corrosion inhibition of the metal surfaces, they also increase the adhesion of lacquers subsequently applied to the surface. In addition, they are able in certain cases to contribute towards improving the properties of metal sheets for cold forming and for deep drawing.
Depending upon the composition of the solution used for phosphating, upon the accelerator used for the phosphating process, upon the process by which the phosphating solution is applied to the metal surfaces and/or upon other process parameters, the phosphate coating on the metal surface is not entirely without interruptions. Thus, more or less large uncoated spots or pores remain, and have to be eliminated or closed by after-passivation to ensure that corrosive influences on the metal surfaces do not have a starting point.
It has long been known that solutions containing chromium salts may be used for this purpose. In particular, the resistance to corrosion of the coatings produced by phosphating is considerably improved by aftertreatment of the surfaces with solutions containing hexavalent chromium.
A major disadvantage of using solutions containing chromium salts is that they are highly toxic. In addition, undesirable bubble formation during the subsequent application of lacquers or other coating materials is observed to an increasing extent.
For this reason, numerous other possibilities have been proposed for the after-passivation of phosphated metal surfaces, including the use of zirconium salts (U.S. Pat. No. 3,695,942, Canadian Pat. No. 966,021), cerium salts (U.K. Pat. No. 1,398,047, German published application No. 23 34 342), polymeric aluminium salts (U.S. Pat. No. 3,852,123, German published application No. 23 25 974), oligo- or polyphosphoric acid esters of inositol in conjunction with a water-soluble alkali or alkaline-earth metal salt of these esters (U.S. Pat. No. 3,957,543, German published application No. 24 03 022) or even fluorides of various metals including TiF.sub.4 and NiF.sub.2 (U.S. Pat. No. 3,895,970, Canadian patent No. 999,220, German published application No. 24 28 065).
Apart from the fact that fluorides are only used in exceptional cases in solutions suitable for after-passivation, because fluoride ions (and sulfate ions) are generally considered to have more of a corrosion-promoting effect, processes of the type in question have never been adopted by industry. Instead, the increasing demands for corrosion prevention have until now only been satisfied by sealing after rinsing with chromate-containing aqueous solutions [cf. W. Rausch, Die Phosphatierung von Metallen, E. Leuze Verlag, Saulgau, Germany (1974)].
Another process for treating metal phosphated surfaces is disclosed in European published patent application No. 85,626. In this process, metal surfaces are treated with aqueous solutions containing trivalent titanium cations and having an acidic pH value, after the phosphating step and before the application of coating materials. The trivalent titanium cation required for the after-passivation solution has to be produced by reduction from tetravalent titanium in a preceding reaction step, which means that the actual after-passivation step has to be preceded by a reduction reaction carried out in correspondingly elaborate reactors. In addition, the solutions disclosed in the above-mentioned application contain anions which have a highly corrosive effect upon parts of the equipment used (such as chloride and sulfate) and/or which are known to interfere with the passivation process (such as fluoride).
For the reasons explained above, it is highly desirable to replace chromium-containing solutions used almost exclusively for the after-passivation of phosphated metal surfaces by non-toxic solutions which do not give rise to bubble formation. However, the development of a universal process workable on an industrial scale for the efficient after-passivaticn of phosphated metal surfaces is a problem which had not until now been solved.